Assays such as heterogeneous immunoassays (i.e., an assay where one component in the liquid phase binds with another component in the solid phase) are widely used for applications in life sciences and diagnostics, and are usually carried out in microwells. In the microwell format, however, long incubation times are typically required. As the affinity reaction proceeds, the concentration of the molecules in the layer of fluid located close to the surface decreases. Diffusion of molecules from the bulk of the solution is then needed to replenish that layer of fluid to allow more binding events to take place. Long incubation times are needed to allow the diffusion of the molecules from the bulk of the solution towards the surface. Recent developments showed that these diffusion-limited reactions take place faster in channels of micrometer dimensions, i.e. in microfluidic devices. One reason for the fast reaction times is attributed to the presence of a flow of fresh solution next to the solid phase. Incubation under flow-conditions in microchips (i.e., microfluidic chips or devices) achieves fast transport of molecules to the surface, and replenishes the layer of fluid close to the surface faster than by the diffusion mechanism.
In microfluidic channels, a small volume of solution (i.e., microliters or less) can sustain a flow sufficient for a fast replenishment of the solution close to the surface for several minutes. These features are quite attractive for the applications of immunoassays, because they result in the consumption of less solution and in faster assays compared to the microwell format. As a result, heterogeneous immunoassays in microfluidic devices have been reported frequently in the scientific literature. In these reports, the lateral dimensions of the channels were typically around 10-200 μm. These dimensions are well suited to benefit from the advantage of microfluidics for immunoassays, but they require the use of magnifying optics and the precise positioning of optics to allow detection of a signal within the channel. These techniques typically require substantial capital equipment that can be both expensive and bulky, thus limiting where and when the detection can take place. Advances in the field that could, for example, reduce costs and/or increase portability would find application in a number of different fields.